Compound semiconductor laser diodes (LD) based on III-V materials such as GaAs have been in use for more than three decades. Current available LDs emit typically in the near infra-red spectral region. The most common applications of these LDs are in two specific areas, the optical-fiber communications and the compact disk (CD) players and CD ROMs. Although the visible LED and LD are highly desirable, in many applications especially in the display areas, they are not available due to the band gap limitation of the existing III-V compound semiconductor materials. In the last few years, red LD based on the quaternary Al--Ga--In--P system has produced light in the spectrum range of 630 to 670 nm region. Table 1 lists the currently available semiconductor LDs and their emitting wavelengths. From Table 1, it is quite clear that two of the most important color sources, blue (.apprxeq.470 nm) and green (.apprxeq.550 nm), are not available.
TABLE 1 ______________________________________ Currently Available Semiconductor Laser Diodes and Their Emitting Wavelengths: Compound Emitting Wavelengths (.mu.m) ______________________________________ AlGaInP 0.65-0.68 Ga.sub.0.5 In.sub.0.5 P 0.67 Ga.sub.1-x Al.sub.x As 0.63-0.90 GaAs 0.90 In.sub.0.2 Ga.sub.0.8 As 0.98 In.sub.1-x Ga.sub.x As.sub.y P.sub.1-y 1.10-1.65 In.sub.0.73 Ga.sub.0.27 As.sub.0.58 P.sub.0.42 1.31 In.sub.0.58 Ga.sub.0.42 As.sub.0.9 P.sub.0.1 1.55 InGaAsSb 1.7-4.4 PbEuSeTe 3.3-5.8 PbSSe 4.2-8.0 PbSnTe 6.3-29 PbSnSe 8.0-29 ______________________________________
To achieve the blue and green LED and LD emissions, it has been necessary to use direct bandgap compound semiconductor materials with band gap sufficiently wide enough (&gt;2-3 eV) to generate the blue or green transitions as shown in FIG. 1. Since the traditional semiconductor materials such as GaAs and InP do not have a sufficiently wide enough bandgap, one approach is to use the so-called wide-bandgap II-VI semiconductor materials such as ZnSe which has a band gap of 2.3 eV. The reason to use ZnSe is that its lattice matches reasonably well to that of GaAs and Ge so thin epitaxial thin films can be made using these readily available commercial substrates. However, the production of blue and green emissions is far from easy. It was not until 1991 with the combination of the development of multi-layer quantum-well heterostructures by the Molecular Beam Epitaxial (MBE) technique and the successful achievement of p-type doping using nitrogen make the fabrication of the injection laser device possible. At present the II-VI LD devices are fabricated on GaAs substrate with ZnMgSSe optical-cladding layers and ZnSSe waveguiding regions and two to three ZnCdSe quantum wells. The LD has operated successfully in cw (continuous wave) mode at 470 nm in liquid nitrogen temperature and at 508 nm at room temperature. Unfortunately the lifetime of cw room temperature operation is very short, and is typically less than one hour. This short lifetime is due to both the high ohmic contact resistivity and the high microstructural defects which act as nonradiative recombination centers at the gain region. The effort to overcome these basic problems has not been successful. A fundamental reason for why these problems exist is that these class of materials being used are simply too soft to tolerate lattice stresses produced by mismatching. Material strength is determined by its intrinsic chemical nature (the bonding strength) and is not changeable.
Another approach to achieve the same blue and green LED and LD emissions is to use the direct bandgap (2.8 eV) of SiC (6H). Indeed, blue LED based on SiC has been produced commercially but the efficiency is very poor due to the high microstructure defects of the SiC devices. In this case, the material is much stronger than ZnSe so that even with such a high density of defects, the device still works. To improve efficiency, the system requires high quality defect-free SiC single crystals to make the substrates. Unfortunately, SiC single crystal can only be grown at extremely high temperatures (&gt;2300.degree. C.) through a vapor transport process in order to achieve reasonable growth rates. This is an extremely difficult process. Current SiC substrates contain undesirable pipe-shaped voids. To improve the current bulk SiC growth technology to produce defect-free single crystal substrates is both difficult and costly. Thus, it is unlikely that SiC LED can be improved with the existing technology to produce efficient blue/green emissions.
A third approach to achieve the same blue and green LED and LD emissions is to use the direct energy wide-bandgap III-V nitride compound semiconductor materials such as AlN, GaN and InN. Again, producing blue and green emissions is difficult. In addition to the difficulty of producing the p-type doping layers, these compounds have another unique problem which does occur in other compound semiconductors. This additional problem is the lack of suitable substrate materials to grow high quality single crystal epitaxial thin films. Nevertheless, in December 1993, Nichia Chemical Co. of Japan has successfully developed a high brightness (100 times that of SiC LED) and high efficiency (3.8%) 450 nm blue LED based on a GaInN composition. However, Nichia's effort has the best performance but it is by no means the first disclosure on GaInN LEDs. There are many prior disclosures on the fabrication of GaInN LED such as U.S. Pat. No. 5,006,908 was issued to Nippon Telegraph and Telephone Co. (NTT) of Japan. In both Nichia and NTT cases, single crystal sapphire (Al.sub.2 O.sub.3) substrates were used to grow the GaN LED thin films because no better substrate materials are available. Since the lattice match between the sapphire substrate and the deposited GaInN film is very poor, the defect concentration is very high (on the order of 10.sup.9 -10.sup.11 dislocations per cm.sup.2). Nevertheless, since GaN is another physically strong material, similar to SiC, even with such a high defect density, the efficiency of these LED devices is still very high. Nichia's result showed that the GaN based compound semiconductor devices are the ideal candidates for LD applications. In late 1994, Cree Research Corporation of North Carolina has successfully fabricated high brightness blue LED based on GaN heterostructure films grown on SiC substrate. Despite the success of these blue LEDs, no one is able to produce any blue LDs based on these III-V nitride compound semiconductor materials.
The primary reason of failing to produce the blue GaN LDs is not because of the film deposition processes nor the impurity doping techniques. It is due to the lack of lattice matching substrates so that high quality GaN single crystal epitaxial films can not be produced. Since there is no lattice match between the GaN and the sapphire substrate, the active film is deposited over a buffer layer which is essentially fine polycrystalline mass. The overgrown active GaN layer can be highly oriented due to the preferential fast growth of those grains with [0001] orientation perpendicular to the surface. The large scattering loss at these grain boundaries is very difficult to overcome by the optical gain to achieve lasing.
III-V nitride compounds having the wurtzite structure which is hexagonal in symmetry, in general, have much smaller lattice constants (a-axis dimension=3.104.ANG. for AlN, 3.180.ANG. for GaN and 3.533 for InN) as compared to all the currently available semiconductor substrates which are all in cubic symmetry. See FIG. 1. Two of the most commonly known wurtzite structure compounds are SiC and ZnO. Both have lattice constants comparatively close to that of the III-V nitrides and both of them are considered and have been used as substrates for the epitaxial growth of nitride thin films. As previously mentioned, the Cree Corporation of North Carolina has succeeded to make a GaN blue LEDs on SiC substrate. However, both SiC and ZnO compounds have serious drawbacks and are not really suitable for this application.
SiC has an a-axis dimension of 3.076.ANG. (Table 2) which is 0.72% smaller than AlN and 3.09% smaller than GaN. To achieve good quality epitaxial thin film growth, it is necessary to have the substrate lattice matched as closely as possible, preferably better than 0.01% and in some cases 0.1%. Clearly, SiC's match is not good for AlN and much worse for GaN. Since AlN already has the smallest lattice constant among the III-V nitride compounds, there is no chance for SiC to get an exact match to any nitride composition. This is one of the reasons why only LED and not LD has been produced with SiC substrate.
In addition to the poor lattice matching, SiC has three additional problems: growth, defects and fabrication. SiC single crystal is produced by physical vapor deposition method at very high temperatures (&gt;2300.degree. C.). The equipment is expensive and the growth process is slow. Moreover, current technology is limited to 30 mm in diameter and the maximum boule length is approximately 50 mm. Secondly, since the growth is invisible, it is not easy to control the growth process and the crystal defects can be very high, including inclusions and hallowed pipe defects. At present, there is no good solution to improve the growth and to eliminate these defects. Thirdly, SiC is a very hard material approaching to the hardness of diamond and it has been used extensively as abrasives. Therefore, wafer slicing and subsequent polishing are very slow processes. In addition, the combination of these problems further adds to the cost of these substrates. Based on these reasons, SiC is not a good substrate for III-V nitride compound semiconductor thin film growth.
Unlike SiC, the ZnO has a totally different set of problems. First, the a-axis lattice constant of 3.2496.ANG. is 4.69% bigger than that of AlN and 2.19% bigger than that of GaN (Table 2). ZnO will not match any of the (Al,Ga)N thin film compositions. But, ZnO does match to the Ga.sub.0.8 In.sub.0.2 N film composition. However, there are serious problems associated with the use of ZnO substrate. The first one is :growth. Even though ZnO has a melting temperature of 1975.degree. C., it can not be grown by the standard Czochralski pulling technique due to the high vapor pressure. Single crystals of ZnO have been produced by physical vapor transport, chemical vapor transport, flux growth and hydrothermal growth. Among them only physical vapor transport and hydrothermal methods have produced crystals greater than 1 cm in dimension. Thus far, the growth rate of all these methods are slow and the crystal size is also small. The crystal can easily develop a twin structure which is also a problem. The second problem is the chemical stability. ZnO crystal sublimes at very low temperatures (&lt;1000.degree. C.) and also reacts with hydrogen at similar temperature range. Hydrogen is a common carrier for the metal organics chemical vapor deposition (MOCVD) process. As a consequence, a ZnO substrate will self-disintegrate at the GaN film deposition temperature. This is pan of the reason why no GaN LED has yet been made successfully on ZnO substrate.
The following is a summary of the state of the art of the blue/green LED and LD technologies and their problems:
(1) Both blue and green LEDs and LDs have been produced based on II-VI ZnSe compound semiconductors. Since these materials are softer than the carbides and nitrites, they suffer rapid device degradation with usage. The materials works well at cryogenic but not room temperatures.
(2) SiC LEDs have been produced commercially but they are not efficient due to high detect density. No LD has yet been produced based on SiC.
(3) High brightness blue LEDs are now produced commercially based on III-V nitride compound semiconductor materials. The nitride thin films are grown on sapphire (Al.sub.2 O.sub.3) substrates and are not true epitaxial films. Nevertheless, long term room temperature operation has been demonstrated. Because of poor lattice match, no LD has yet been produced. Blue III-V nitride LEDs are also produced using SiC substrate, again no LD has yet been produced. No nitride LED nor LD has successfully been made on ZnO substrates.